EVOLUTION OF THE HUMAN HAND. 461 



The first of these functions appears to be essentially connected with 

 the habit of walking on the sole of the foot — plantigrade locomotion 

 — and not on the knuckles or toes — digitigrade locomotion. Another 

 method of climbing exists which is common to the rodents and the 

 cats. In these animals the act of climbing depends upon the develop- 

 ment of claws sufficiently long, strong and sharp to be attached like 

 hooks to the roughened surfaces upon which the animal climbs and 

 thus to support the body. In such forms the modification is of a super- 

 ficial feature of the body structure and is probably a secondary func- 

 tion, the claws having been developed in connection with habits of 

 seizing prey rather than of climbing. There is here no essential 

 modification in the anatomical relations of the various parts of the 

 limb, and it is inconceivable that any such subsequent development 

 should be connected with this form of climbing organ as is presented 

 in the limbs of the anthropoid apes and man. 



In the plantigrade animal, on the other hand, the disposition of 

 the limb is such that when the weight of the body is thrown upon it 

 the toes tend to be thrust apart even when the foot is resting on a flat 

 surface, and to be forced into a concave shape when pressed upon a 

 rounded object. It is probable that a fair degree of development in 

 the joints of the limbs had taken place in both flexion and separation 

 before they were used for the purpose of climbing. But flexibility and 

 separability of the digits form only the intial step in the process by 

 which adaptation to an arboreal life was perfected. The second — 

 and beyond all other changes important — modification consisted in the 

 structural opposition of one digit to the remaining group. This 

 differentiation occurs also in the lizards, e. g., the chameleon, and in 

 the birds, under similar conditions of climbing and perching; but in 

 connection with such specialization of the limbs in other regards and 

 such modification of the body system as a whole that important service 

 in the evolution of intelligence was precluded. 



In the production of opposition changes took place in the hind 

 limbs first and most generally, since all species in which the thumb 

 is opposed possess opposable great toes also — except in the single case 

 of man — while many species occur in which opposition is presented 

 by the hind limbs alone. In this adaptation of the foot to climbing 

 three structural changes were effected — the parts of the limb became 

 more flexible, the joints more widely separable, and the great toe, as 

 has been said, opposed to the group formed by the remaining digits. 

 All these are important features in rendering the limb a more efficient 

 tool. 



For the development of those peculiar functions which char- 

 acterize the human hand, however, a further change in the use of the 

 fore limb was necessary, by which it was relieved from participation in 

 the support of the body and in primary locomotion. This relief must 



